1. Field of the Invention
The present invention relates to systems and methods for performing eye tracking, and, more particularly, to such systems and methods for controlling the functions of an eye tracker during measurement and correction of aberrations in a visual system.
2. Description of Related Art
Optical systems having a real image focus can receive collimated light and focus it at a point. Such optical systems can be found in nature, e.g., human and animal eyes, or can be manmade, e.g., laboratory systems, guidance systems, and the like. In either case, aberrations in the optical system can affect the system""s performance. By way of example, the human eye will be used to explain this problem.
A perfect or ideal eye diffusely reflects an impinging light beam from its retina through the optics of the eye, which includes a lens and a cornea. For such an ideal eye in a relaxed state, i.e., not accommodating to provide near-field focus, reflected light exits the eye as a sequence of plane waves. However, an eye typically has aberrations that cause deformation or distortion of reflected light waves exiting the eye. An aberrated eye diffusely reflects an impinging light beam from its retina through its lens and cornea as a sequence of distorted wavefronts.
There are a number of technologies that attempt to provide the patient with improved visual acuity. Examples of such technologies include remodeling of the cornea using refractive laser surgery or intra-corneal implants, adding synthetic lenses to the optical system using intra-ocular lens implants, and precision-ground spectacles. In each case, the amount of corrective treatment is typically determined by placing spherical and/or cylindrical lenses of known refractive power at the spectacle plane (approximately 1.0-1.5 cms anterior to the cornea) and literally asking the patient which lens or lens combination provides the clearest vision. This is an imprecise measurement of true distortions in the reflected wavefront because (1) a single spherocylindrical compensation is applied across the entire wavefront; (2) vision is tested at discrete intervals (i.e., diopter units) of refractive correction; and (3) subjective determination by the patient is made in order to determine the optical correction. Thus conventional methodology for determining refractive errors in the eye is substantially less accurate than the techniques now available for correcting ocular aberrations.
Various embodiments of a method and system for objectively measuring aberrations of optical systems by wavefront analysis have been disclosed in commonly owned application Ser. No. 09/566,668, xe2x80x9cApparatus and Method for Objective Measurement and Correction of Optical Systems Using Wavefront Analysis,xe2x80x9d filed May 8, 2000, which is hereby incorporated by reference herein. In this invention, an energy source generates a beam of radiation. Optics, disposed in the path of the beam, direct the beam through a focusing optical system (e.g., the eye) that has a rear portion (e.g., the retina) that provides a diffuse reflector. The beam is diffusely reflected back from the rear portion as a wavefront of radiation that passes through the focusing optical system to impinge on the optics. The optics project the wavefront to a wavefront analyzer in direct correspondence with the wavefront as it emerges from the focusing optical system. A wavefront analyzer is disposed in the path of the wavefront projected from the optics and calculates distortions of the wavefront as an estimate of ocular aberrations of the focusing optical system. The wavefront analyzer includes a wavefront sensor coupled to a processor that analyzes the sensor data to reconstruct the wavefront to include the distortions thereof.
A perfectly collimated light beam (i.e., a bundle of parallel light rays, here a small-diameter, eye-safe laser beam) incident on a perfect, ideal emmetropic eye, focuses to a diffraction-limited small spot on the retina. This perfect focusing is true for all light rays passing through the entrance pupil, regardless of position. From the wavefront perspective, the collimated light represents a series of perfect plane waves striking the eye. The light emanates from an illuminated spot on the retina as wavefronts exiting as a series of perfect plane waves, which are directed onto a wavefront analyzer for measuring distortions from ideality.
One problem with the sensing of such wavefront data is the natural eye movement that occurs during an exposure. Multiple exposures may be used to check for improper eye alignment or eye movement during individual exposures. However, often eye movement during exposures cannot be analyzed successfully by acquiring multiple exposures.
Following measurement of the eye aberrations, a patient may elect to undergo corrective laser surgery, performed, for example, by laser ablation of portions of the corneal surface to achieve a calculated shape for improving visual acuity. In this case it is also desirable to account for eye movement during surgery while delivering laser shots to the cornea. Given an eye tracker apparatus as part of the ablation system, it is also desirable to account for any object that may temporarily obscure the field of vision of the tracker.
It is therefore an object of the present invention to provide a system and method for tracking eye movement during measurement of ocular aberrations.
It is a further object to provide a system and method for tracking eye movement during laser surgery to correct ocular aberrations.
It is another object to provide a system and method for detecting an object obscuring a field of view of a tracking system.
It is an additional object to provide such a system and method for responding to the obscuring object.
It is yet a further object to provide such a system and method for aborting a surgical procedure under certain predetermined conditions of the tracking system.
It is yet another object to provide such a system and method for temporarily halting a laser surgical procedure during an obscuring of the tracker system.
These and other objects are achieved by the present invention, a system and method for controlling an eye movement tracker. The method comprises the step of monitoring a plurality of positions of an eye at a predetermined rate by following a predetermined eye feature using the tracker. An optical beam is sent into the eye, and an intensity of a reflected beam from the eye is sensed at each position.
If the intensity of the reflected beam fluctuates from a predetermined acceptable intensity range, the tracker is returned to a frozen position. The frozen position comprises a most recent position at which the intensity lay within the intensity range.
The method further comprises steps for freezing the tracker if the noise in the signal exceeds a predetermined acceptable maximum noise level and for counting a number of times the tracker is frozen. The procedure is aborted if the tracker is frozen repeatedly and for a time exceeding a predetermined maximum acceptable time.
The system of the present invention comprises means for performing the above-recited steps, including a processor and software means for performing the required calculations.
The features that characterize the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description used in conjunction with the accompanying drawing. It is to be expressly understood that the drawing is for the purpose of illustration and description and is not intended as a definition of the limits of the invention. These and other objects attained, and advantages offered, by the present invention will become more fully apparent as the description that now follows is read in conjunction with the accompanying drawing.